More detailed "make your own externs" documentation proposal

[AnotherKamila]

It would have saved me a lot of time if there was documentation about the interface that P4 expects from externs. I propose adding the following paragraph to the wiki, under "Adding your own externs", before the last paragraph:

Externs must be pipelined: P4 may input data in any cycle, not only after the previous computation has finished. In addition to the reset, clock, input, and output signals, there is also an extra input bit valid_in and an extra output bit valid_out. The valid_in bit will be high when we have valid data to compute with (i.e. if it is not set, we can ignore that input). The valid_out bit should be set whenever the result of a valid input (i.e. of data that was input when valid_in was set) is ready -- this bit signals to P4 that it can take the data and resume execution of the caller pipeline.

I can make the change, if you okay it.

[sibanez12]

This sounds good. Please do add in this paragraph to the documentation. I can supplement with a few more details after you add this in.

Thanks :)

[AnotherKamila]

Done.

It might make sense to split out the externs info from the "Wokflow Overview" wiki page, as by now it is very long, but that's for you to consider.